Concrete slipform pavers are known. Specifically, such pavers include a "tractor" and a "paving kit".
Regarding the tractor, most concrete slipform pavers include a tractor which is comprised of a rectilinear frame which straddles the concrete roadway or runway while they are paved. This frame is propelled and supported on either end by side bolsters and crawler track(s). The frame supports a diesel engine driven hydraulic power unit which supplies power to the tractor and paving kit.
The paving kit is typically suspended below the tractor frame by mechanical connections. The paving kit takes its hydraulic power from the power unit on the tractor. The tractor and paving kit comprising the slipform pass over the concrete placed in its path in a relatively even and level mass that can be conveniently paved. During this slipform process the tractor attached paving kit spreads the concrete dumped in the path of the paver, levels and vibrates it into a semi-liquid state, then confines and finishes the concrete into a slab with an upwardly exposed and finished surface. Sideforms mounted to the side of the slipform paving kit confine the sides of the slab during the paving process leaving a vertical edge on both sides of the slab once the slipform paving kit has passed.
The tractor typically has either two or four crawler tracks supporting and propelling the frame and attached paving kit. Other kits can be attached to these tractors such as kits for conveying and spreading concrete and trimming and spreading base materials. For the purposes of this description, we will focus on the paving kit used for slipform paving. These "two track" or "four track" pavers must be separately considered to fully understand this invention.
With respect to both two and four track pavers, it is known to telescope the tractor frame normal to the direction of the paving movement. This telescoping normal to the direction of the paving movement enables the frames to span different widths of pavements within the limits of the telescopic extensions. Once these telescopic extensions limits are reached, a fixed frame extension can be added to one or both sides of the telescopic tractor frame for further extension. Despite the telescopic ability of the frame, the process is still a relatively complex and time consuming operation. Adding a fixed frame extension(s) significantly increases the complexity and difficulty of the frame width change. What is more time consuming is changing the paving kit to span different widths of pavements. The paving kit comes in discrete sections. Each of these kit sections must be individually inserted or removed and the bolting connections between each section made or broken. It is not uncommon for more than one day to be consumed in such kit adjustments.
A two track paver tractor has one crawler track located on each side of the machine. Each crawler track at its point of attachment to the frame includes a hydraulic cylinder, one at each corner. Thus there are two cylinders per side. Typically, the front and rear cylinders on a side are connected to each other by means of a side bolster, which in turn is connected to the side of the tractor frame. On four track paver, each crawler track at its point of attachment to the side of the tractor frame includes a pivot arm with attached hydraulic cylinder. The pivot arm pivots about a vertical axis and allows the crawler track to pivot with respect to the corner of the tractor frame. The front and rear cylinders on a side are connected to each other by means of a side bolster which in turn is connected to the side of the tractor frame. The pivot arms are actually attached to each end of the side bolster. The pivoting movement is normally used to reposition the crawler tracks in relation to the machine. When transport is about to occur, pivoting occurs between a first disposition where the frame straddles pavement being placed to a second disposition where the crawler is disposed for transport parallel to the length of the frame for transport. These frame supporting hydraulic cylinders can raise or lower the tractor and paving kit relative to the crawler tracks.
The operation of a paver can be simply stated. Typically, a system of grade level reference wires are strung adjacent and parallel to the roadway or runway grade being constructed. Sensors with wire feelers, one located at each corner of the machine attached to the frame, follow the present grade wires. The leveling sensor on the frame independently adjusts the height of the frame relative to each of the crawler tracks at each corner of the frame. This adjustment occurs through the frame supporting cylinders at each of the crawler tracks at each of the four corners of the machine. The paving kit suspended from the frame thus is continually adjusted to maintain a preset elevation disposition relative to the wires as paving occurs.
Two track pavers have the advantage of being more suitable for small jobs because they are simpler with fewer moving parts and more convenient to transport. They are easier to transport because the crawler track length can be limited to an overall length matching the maximum load width dimension which can be legally transported on the highway without a pilot car (in the U.S.A. this is 12' or 3.657 M).
Disadvantages exist with such two track machines. Access to the sideforms attached to the paving kit is obstructed for cleaning the sideforms of accumulated concrete and for inserting side tiebars and dowel bars. Further, since we are limited as to how long the crawler tracks can be in order to conveniently transport the machine, the entire weight of the machine is borne by the two crawler tracks of a limited width. Consequently, high ground pressure is unavoidable, which allows the tracks to sink into soft subgrade or trackline (the grade immediately adjacent to either side of the slab where the crawler tracks travel) which can adversely affect the ability of the machine to hold a preset slab thickness or level. Another disadvantage of the two track machine, because the crawler is relatively short, is that the rear hydraulic cylinder for leveling is so close to the rear of the slipform pan where the final grade of the concrete slab is established. Ideally, as is the case with a four track machine, the front and rear hydraulic leveling cylinder should be roughly equidistant from the rear of the slipform pan to help average the correction for grade deviations. This feature has been understood to allow smoother slabs to be placed as compared to a two track tractor on most tracklines. Finally, loading of such machines onto a transporting flat bed truck requires placement of ramps with the two tracks climbing their own ramps onto the transporting trailer.
Four track pavers have the advantage of the front and rear hydraulic leveling cylinder being roughly equidistant from the rear of the slipform pan which helps average the correction for grade deviations as described above. Additionally, four track pavers allow the supporting crawler tracks to straddle the paving kit at the sides of the paver. One crawler track is spaced forward at the side of the machine, one crawler track is spaced rearward at the side of the machine, and the paving kit is exposed at the paving kit sideforms between the crawler tracks. Thus the sideforms are easily accessible and easier to clean. Moreover, joint forming and bar inserting apparatus is more easily suspended and accessible. For example, the insertion of reinforcing bars into the side of a paved slab is not as easily accomplished in the case of a two track paver because the tracks interfere with the required placement.
The greatest disadvantage of a four track paver occurs when the machine is configured for transport from one job site to another job site. Two problems are commonly encountered. The first problem is a series of required changes in the machine configuration for transport. The second problem is the length of the transported load.
A common expedient regarding machine configuration for transport is to pivot the four supporting crawler tracks from the frame straddling support position to where the crawlers are disposed for transport parallel to the length of the frame and within the width of the frame for transport in the travel position. First, the four crawler pivot arms are disconnected from where they are attached and secured into position to the front and rear of the tractor frame. Thereafter, the frame is supported and the crawler tracks are retracted off the ground so the pivot arm and crawler may be pivoted 90.degree. so both the tracks and pivot arms within the width of the pavement spanning frame. Once this is done the pivot arms must be resecured in the transport position to hold the pivot arms in place. Hydraulic plumbing must also be modified to allow the machine to walk onto a transport trailer under its own power. Unfortunately, conventional four track paving machines when configured for transport require substantial alteration of both the frame and the paving kit before transport can occur.
Secondly, such machines when configured for the transport disposition have excessive length. In preparation for transport, the crawler tracks are swung on their pivot arms relative to the frame. Such swinging occurs from the position where the crawler tracks span the paving dimension of the frame to a position where the crawler tracks are in line with the dimension of the frame normal to the direction of the paver travel. When the crawler tracks are in line normal to the paving dimension of the machine, the crawler tracks add to the transport length of the machine. An example utilizing dimensions can be helpful.
Take the case of a four track paver configured for paving two lanes. Each lane is 12' for a total paving width of 24'. Each crawler track, when swung from a position which enables the frame to straddle the paving to a configuration crawler track is aligned parallel to the frame adds 12' to the length of the frame. It will thus be understood that the combination of the leading tracks and the trailing tracks adds 24' to the length of the frame. Consequently, when the four track paving machine is configured for transport, the total machine length exceeds 48 feet. When the paving width is 30 feet, a 54 foot transportable load is required.
Most transporting flat bed or single drop deck trailers are in the 40 foot range. Transporting a load over 48 feet is of considerable inconvenience. Further, the preparation of such transport is tedious--time intervals up to and exceeding one day are frequently required to prepare such a machine for transport.